Conventionally, silica has been used as a reinforcing filler in order to enhance the safety and the fuel economy of tires. Since use of only silica does not provide a sufficient effect, a silane coupling agent that links silica with a rubber component has been used in combination with silica.
To effect a reaction between a silane coupling agent and silica, an alkoxy group or the like bonding to a silicon atom in the silane coupling agent needs to be hydrolyzed to form a silanol group. However, since hydrolysis of the alkoxy group or the like does not proceed in a short time period, in practice, hydrolysis of the alkoxy group or the like does not proceed sufficiently in the step of mixing the rubber composition. As a result, the reaction rate between the silane coupling agent and silica is lowered, failing to optimize the performance of silica.
As methods for solving the above problem, Patent Documents 1 to 3 disclose providing rubber compositions respectively blended with various sodium salts, various potassium salts, and boric acid. However, the use of a hydroxy acid or itaconic acid is not studied in detail.
Patent Document 1: JP 2001-247718 A
Patent Document 2: JP 2005-232295 A
Patent Document 3: JP 2007-77322 A